[S-CG54] Volcanoes in the sea

[SCG54-02] A review of seismic studies in the Izu-Bonin Arc: Uncovering crustal formation processes related to island arc volcanism

★Invited Papers

*Shuichi Kodaira¹ (1.Research Institute for Marine Geodynamics, Japan Agency for Marine-Earth Science and Technology)

Keywords:Izu-Bonin arc, Island arc volcano, Crustal structure, Active-source seismic survey

JAMSTEC conducted an extensive active-source seismic survey project covering the entire Izu-Bonin arc in the early 2000s, for approximately a decade. This initiative has yielded novel seismological insights into the deep structure of volcanoes, unveiling the formation and evolutionary processes of the volcanic arc crust. Results from a 1000 km-long profile along the volcanic front reveal a significant volume of felsic-to-intermediate component crust, characterized by a compressional wave velocity (Vp) ranging from 6.0 to 6.8 km/s beneath basaltic volcanic centers along the current volcanic front. A similar variation in the felsic-to-intermediate component crust has been identified in the rear-arc, believed to have separated from the volcanic front after the Oligocene age. These findings suggest that the predominant composition of the arc crust, primarily felsic-to-intermediate, was established before the separation of the rear-arc from the volcanic front, likely occurring during the Oligocene age. Conversely, seismic data acquired from the fore-arc region indicates distinctive structural features, showing a crust thickness of less than 10 km thick. Furthermore, velocity-depth profiles in the fore-arc closely resemble the characteristics of typical oceanic crust. Petrological studies in the fore-arc region propose the formation of oceanic crust through Boninitic volcanism during the initial stages of subduction. This hypothesis gains robust support from seismic structures, revealing a remarkably thin crust beneath the Bonin Ridge in the fore-arc. The results of these studies elucidate the crustal evolution process of the Izu-Bonin volcanic arc from its very early stages to the present, providing crucial fundamental data for understanding crustal formation processes related to island arc volcanism.